(1) Field of the Invention
The present invention relates to a method and apparatus for the self-diagnosis of the air leakage in a control system of an internal combustion engine. More particularly, the present invention relates to a method and apparatus in which the leakage of air into a suction system of the engine is self-diagnosed and the detection value of the flow quantity of sucked air is corrected based on the result of the self-diagnosis.
(2) Description of the Related Art
There has been known a control system for electronically controlling the quantity of a fuel supplied into an engine, in which an sucked air flow quantity, which is a quantity of the state of sucked air participating in the quantity of air sucked into the engine, is detected by an air flow meter and the fuel supply quantity is variably set based on this detected sucked air flow quantity and the revolution speed of the engine (Japanese Unexamined Patent Publication No. 60-153446).
In an internal combustion engine, air purified by an air cleaner is supplied into the engine through a suction duct and a suction manifold and the sucked air flow quantity is controlled by a suction throttle valve. However, in the system for electronically controlling the fuel supply quantity based on the sucked air flow quantity in the above-mentioned manner, if air leaks into the suction system from a portion other than the prescribed air introduction system, it is apprehended that the following problems will arise.
More specifically, in a fuel supply control apparatus generally called "L-Jetro System" where the flow quantity of air is measured and the fuel supply quantity is electronically controlled based on the measured sucked air flow quantity, the structure is arranged so that all of sucked air is introduced into the engine through the air flow meter. However, if there is present leaking air sucked into the engine without being measured by the air flow meter (for example, air leaking from a clearance of a connecting part between parts constituting the suction system disposed downstream of the air flow meter), the fuel supply quantity is set at a quantity smaller than the quantity corresponding to the actually sucked air quantity, and the air-fuel ratio becomes leaner than the target air-fuel ratio. Accordingly, if the leakage of air takes place, misfire is caused and the unburnt gas is burnt in a catalyzer, resulting in occurrence of burnout of the catalyzer.
Furthermore, in the case where an ignition system for performing electronic control according to the ignition timing set based on the basic fuel supply quantity set by the sucked air flow quantity is disposed, if the basic fuel supply quantity comes to include an error because of occurrence of the leakage of air, the set ignition timing deviates from the value required by the engine and the ignition controllability is degraded.
Therefore, it is necessary to dispose an apparatus capable of self-diagnosing the leakage of air and making a correction according to the leakage quantity when the leakage of air occurs. However, according to the conventional technique, even if the air-fuel ratio is detected based on the oxygen concentration in the exhaust gas of the engine, since it is impossible to distinguish whether or not the mean deviation of the air-fuel ratio detected is due to the leakage of air into the suction system of the engine, once the mean deviation of the air-fuel ratio is detected, all of electric appliances participating in the control of the air-fuel ratio (a control unit, an air flow meter, a fuel injection valve and the like) are exchanged with new ones.
More specifically, in some electronically controlled fuel supply apparatus, the air-fuel ratio in the air-fuel mixture sucked in the engine is detected through the oxygen concentration in the exhaust gas, and an air-fuel ratio feedback correction function of performing the feedback correction of the basic fuel supply quantity so that the detected air-fuel ratio is brought close to the target air-fuel ratio is given. This function is adjusted so that the correction quantity by the air-fuel ratio feedback correction is minute at the initial stage. If a large correction by the air-fuel ratio feedback control becomes necessary afterward, when the correction is one for increasing the quantity of the fuel, there is a possibility that the detected value of the sucked air flow quantity is made smaller than the true value because of the leakage of air. In this case, however, the deviation cannot be distinguished from the deviation of the air-fuel ratio which is due to the degradation of supply characteristics of a fuel injection valve for supplying the fuel or the trouble of the air flow meter the pressure regulator for adjusting the fuel supply pressure. Therefore, it is necessary to exchange all of the related parts and find out the part causing the deviation. Accordingly, the maintenance characteristics for coping with the leakage of air are poor.